Leishmania parasites cause a spectrum of devastating diseases known as leishmaniasis. A critical step in Leishmania infection is the differentiation from replicative, non-virulent procyclics to non-replicative, highly virulent metacyclics. Our long term goal is to identify intracellular molecules that regulate this transition (termed metacyclogenesis) and understand their mode of action. In mammals and fungi, sphingolipid (SL) metabolites are vital mediators of apoptosis, endocytosis, growth, and differentiation. Our recent studies indicate SLs also play important roles in Leishmania infection: 1) degradation of SLs is the major route to produce ethanolamine, which is essential for metacyclogenesis;2) besides ethanolamine production, certain SL metabolites may also serve as negative regulators of virulence. This proposal will test the hypothesis that intracellular levels of SL metabolites control Leishmania differentiation. Specific aims include: 1) to evaluate the effects of exogenous SL metabolites on metacyclogenesis and virulence;2) to determine the intracellular levels of SL metabolites during Leishmania growth and differentiation. Successful completion of these aims will reveal a previously unrecognized role of SL metabolism in Leishmania infection. Future work includes the identification of SL- binding proteins and the characterization of a SL-mediated signaling pathway in Leishmania parasites. Understanding the role of SLs in metacyclogenesis will provide fundamental insights into the regulation of Leishmania differentiation, a process that is central to the infectivity and virulence of this parasitic protozoan .Leishmania parasites cause a spectrum of devastating diseases in humans known as leishmaniasis, which infect 10-12 million people worldwide. This proposal aims to investigate the roles of a subset of lipid molecules in Leishmania infection. Successful completion of the proposal will help reveal the mechanism of pathogenesis in these medically important (yet often neglected) pathogens.